The present invention relates generally to telecommunications systems and, more particularly, to a telecommunications system for carrying voice over packet-switched networks.
Asynchronous Transfer Mode (ATM) represents a standard for transmitting voice, data, and video signals at very high speeds (25 Megabits/second and higher). The increasing deployment of ATM networks, particularly on customer premises, has created a need to provide high quality transport, switching, and call processing of voice communications over a packet network with minimal delay. The delay must not exceed that currently permitted for existing Private Branch Exchanges (PBXs), and the voice quality must be at least as good as such systems to allow a seamless upgrade from present day centralized PBXs to ATM distributed voice services.
Conventional network architectures for carrying voice over ATM have focused on large centralized or distributed voice switching networks. Two such architectures are described in Ron A. Spanke and J. Mark Adrian, "ATM Composite Cell Switching For DSO Digital Switches," XV International Switching Symposium, pp. 268-272, 1995, and Roy Mauger and Simon Brueckheimer, "The Role of ATM in 64 kb/s Switching and Transmission Networks," XV International Switching Symposium 1995, pp. 87-91. These architectures, however, are designed for large switching systems, such as for central office applications. They suffer when used for distributed voice-over-ATM systems within a customer's premises or campus environment due to blocking limitations, bandwidth inefficiency, and trunking effects.
The main components of such systems are access and switching units and the ATM OFFICES network or switch. The access and switching units provide two functions: (1) switching voice calls from one channel or trunk to another channel or trunk attached to the same unit, and (2) multiplexing voice calls from a channel or trunk into ATM cells to be sent through the ATM network to a channel or trunk attached to a destination access and switching unit. A channel includes any type of voice or communications channel, for example, the H. 320 video at 384 Kbits/sec.
The destination access and switching unit demultiplexes the voice calls onto the appropriate channel or trunk. A channel can be, for example, a DSO single voice channel, which is a 64 kilobit/second, PCM-encoded digital signal converted from an analog voice signal, and a trunk can be, for example, a T1 or E1 facility, which carries multiple DS0s.
An ATM virtual channel is a packet-switched connection between two points using a logical connection consisting of a set of header and port switching translations at every node through which the ATM cell passes. The voice capacity of an ATM virtual channel depends on the number of DS0 voice channels it can carry, which, in turn, depends on the format used to multiplex voice calls into ATM cells.
The key features of an ATM network carrying voice traffic include: (1) voice channel (DS0) switching capacity, (2) ATM bandwidth efficiency, and (3) equipment cost and utilization. To design such a network, engineers use simplifying assumptions to ensure the network meets the needs for all conditions. For one assumption, all ATM virtual channels in the network have the same DS0 voice channel capacity, all access links carrying virtual channels to the ATM network have the same capacity, and each access link can support the same number of virtual channels.
The worst case routing scenario uses the maximum number of virtual channels while carrying the lowest number of voice channels. With N access and switching units, the worst case scenario occurs when only one channel from an access and switching unit is destined for each of the (N-2) other access and switching units, and all the remaining channels are destined for the (N-1)th access and switching unit. Other routing scenarios can use as many virtual channels as this scenario, but none uses more. This scenario requires that the maximum number of channels, n, that can be accommodated without blocking a single access and switching unit under the worst routing scenario is: EQU n=L.sub.max -(T.sub.min -1)(N-2), (1)
where L.sub.max is the maximum number of channels that can be carried by the trunking scheme on a given size link and T.sub.min is the minimum number of channels carried by a trunking scheme between an access and switching unit and the ATM network.
The maximum network capacity is found by multiplying n (from equation (1)) times the number of access and switching units (N), and optimizing. Thus, the number of access and switching units in the largest capacity system, N.sub.large, is: ##EQU1##
The number of DS0 voice channels supported on an access link in this largest system, n.sub.large, is: ##EQU2## Equations (1)-(3) assume full mesh connectivity of access and switching units because any call connected to an access and switching unit may be destined for any other access and switching unit.
Equation (1) forces system designers to contend with the fact that the number of voice channels supported at an access and switching unit decreases as the number of access and switching units, N, increases. This causes designers to overengineer access and switching units, which in turn increases equipment and system costs.
As a concrete example, each access link from the access and switching unit to the ATM network could be an STS-3c with L.sub.max =2112 and T.sub.min =48. Using these values in equation (2) yields twenty-three access and switching units in the largest possible system. Each access link can support 1103 DS0 voice channels, so the total system capacity is 25,369 DS0 voice channels. A typical PBX system in a distributed environment, on the other hand, may want only 200 channels per distributed location, so a typical twenty-three node network would only require a total system capacity of 4600 DS0 xvoice channels.
Therefore, the conventional architecture does not provide an optimal design for replacing PBX systems with voice-over-ATM networks in a distributed environment. The results cause significant overengineering of access links to support the required connectivity. Therefore, few distributed PBX systems can be created with the conventional voice-over-ATM architecture.